Ultra-high voltage metal-oxide-semiconductor field effect transistors (MOSFET) were typically fabricated with coplanar drain and source regions. FIG. 1A shows an ultra-high voltage MOSFET device 100 in the prior art. Device 100 is formed on a p-type substrate 101, and another p-layer 113 is epitaxially grown on substrate 101. High-voltage p-well 115 is adjacent high-voltage n-well 103 in the epitaxially grown p-layer 113. N+ source 117 is positioned in the high-voltage p-well 115, and N+ drain 105 is positioned in high-voltage n-well 103. Gate dielectric 111 and gate electrode 110 extend from over the N+ source 117 to over a portion of the field oxide 107. Device 100 also includes P+ pickup region 119 located in the high-voltage p-well 115. Applying a positive voltage to the gate electrode 110 induces a current to flow through the channel from the N+ source 117 into the high-voltage n-well 103, which current is collected at the N+ drain 105.
A problem with this type of ultra-high voltage MOSFET is that it cannot maintain a low on-resistance when a high voltage is applied on the ultra-high voltage MOSFET. The on-resistance affects the power transformed into heat as the current travels through the device. The greater the on-resistance of the device, the less efficient the device. Accordingly, it is desirable to reduce this resistance as much as possible for a more efficient device.
FIG. 1B illustrates another device 150 known in the prior art that is designed to mitigate this problem. Device 150 is similar to device 100 of FIG. 1A, wherein like reference numerals are used to refer to like elements, except field ring 109 has been added. Field ring 109 works to reduce the surface electrical field and improves the depletion capability of the drift region. As a result, the doping concentration of the drift region can be increased and the on-resistance of the device 150 can be reduced compared to that of device 100.
The breakdown voltages of prior art device 100 (shown in FIG. 1A) and the prior art device 150 (shown in FIG. 1B) are still not satisfactory. As is known in the art, devices can only be operated at voltages lower than the respective breakdown voltages. When a voltage greater than the breakdown voltage is applied on devices such as device 100 and device 150, catastrophic and irreversible damages occur to the devices, rendering the devices commercially useless and requiring the devices to be replaced. Accordingly, increasing the breakdown voltage is highly desirable. An improved ultra-high voltage MOSFET is thus needed for a reduced on-resistance and a further increased breakdown voltage.